The present inventor has determined an understanding that the type of smoke produced in various pyrolysis and combustion circumstances is different. Fast flaming fires tend to produce a very large number of very small solid particles which may agglomerate into random shapes to form soot. In contrast, the early stages of pyrolysis tend to produce a much smaller number of relatively large liquid particles (of high boiling point), typically existing as aerosols that may agglomerate to form larger, translucent spheres.
The present inventor has also determined an understanding that the detection of relatively large particles which slowly increase in quantity over an extended period of time would typically indicate a pyrolysis or smouldering condition, whereas the detection of numerous small particles arising quickly and without earlier pyrolysis or smouldering could indicate arson involving the use of accelerants.
The present inventor has also determined an understanding that dust particles are generated by the abrasion or non-thermal decomposition of natural materials or organisms in the environment and that such particles are in general very large compared with smoke particles.
The present inventor has also determined an understanding of the following:
Conventional point type smoke detectors are primarily designed for ceiling installation in a protected area. These detectors have relatively low sensitivity and have difficulty in detecting the presence of unwanted pyrolysis where large volumes of air pass through the area being monitored, thus diluting the ability for the detector to sense the presence of unwanted pyrolysis.
To overcome these disadvantages, highly-sensitive aspirated smoke detectors where developed, and are often deployed on ducts for the purpose of monitoring an area. These detectors provide a measure of sensitivity some hundred times greater than convention point detectors. These aspirated systems employ suction pressure via an air pump and also employ a dust filter to reduce unwanted dust pollution from soiling the detector or from being detected indistinguishably from smoke and causing the triggering of a false alarm.
The smoke detector preferably employed in an aspirated system is a nephelometer. This is a detector sensitive to many sizes of particles, such as the many smoke particles produced in fires or during the early stages of overheating, pyrolysis or smouldering.
Optical type smoke (or airborne particle) detectors of the prior art typically use a single light source to illuminate a detection zone that may contain such particles. The use of two light sources has been proposed for some detectors. A proportion of this light may be scattered off the particles toward a one or more receiver cells (or sensors). The output signal(s) from the receiver cell(s) is used to trigger an alarm signal.
Other detectors use a laser beam, providing a polarised monochromatic light source, typically in the near infrared wavelength. These detectors, however, are not considered to be true nephelometers as they are prone to being overly sensitive to a particular particle size range at the expense of other size ranges.
The disadvantage suffered by the above detectors is their relative insensitivity to very small particles characteristic of early pyrolysis and incipient fires, as well as certain fast flaming fires.
Ionisation smoke detectors, on the other hand, utilise a radioactive element such as Americium, to ionise the air within the detection chamber. These detectors are relatively sensitive to very small particles produced in flaming fires, but relatively insensitive to larger particles produced in pyrolysis or smouldering. They have also been found relatively prone to draughts, which serve to displace the ionised air within the detection chamber and thus trigger a false alarm. This places a practical limit on their useful sensitivity.
Other smoke detectors have used a Xenon lamp as a single light source. The Xenon lamp produces a continuous spectrum of light similar to sunlight, embracing ultraviolet, visible and infrared wavelengths. Use of this light source can detect all sizes of particles and the detectors produce a signal that is proportional to the mass density of the smoke, which is characteristic of a true nephelometer. However, the type of fire cannot be characterised because the particular particle size cannot be discerned. The Xenon light also has only a relatively short life-span of some 4 years and its light intensity is known to vary, which affects the sensitivity.
The present inventor has also realised that in order to provide a wide output range in sensitivity, prior art detectors provide an analog to digital converter (ADC) used to apply the smoke level data to a microprocessor. With careful design, substantially all of the capacity of the ADC is used to represent the maximum smoke level, such as (typically) 20%/m. ADC's operating at 8-bit resolution are useful, whereas a 10-bit or larger ADC's are more expensive and require larger microprocessors. A 10-bit ADC has been found to allow this 20%/m level to be divided into 1024 steps, each step representing an increment of 20/1024=0.02%/m. So the steps are 0, 0.02, 0.04, 0.06, etc, with no opportunity for finer increments. At low smoke levels this is considered a very coarse resolution, making it difficult to set alarm thresholds finely. However at high smoke levels, a resolution of 0.02%/m is unnecessary—the ability to set an alarm threshold at 10.00%/m or 10.02%/m for example, has little if any benefit. So the resolution of the prior art detectors is considered too coarse at low smoke levels and too fine at high smoke levels.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.
An object of the present invention is to provide a particle detection apparatus and method(s) which enable an improved detection, discrimination and/or analysis of particles, pyrolysis, smouldering and/or flaming events and dust, thus providing a corresponding improvement in fluid-borne particle detection.
A further object of the present invention is to provide a particle detection apparatus and method(s) suitable for use with ducts or as a stand-alone detector and/or monitor.
A still further object of the present invention is to alleviate at least one disadvantage associated with the prior art.